The exhaust gas of internal combustion engines, in particular vehicle engines, contain components which are still combustible (such as carbon monoxide and unburnt hydrocarbons) and oxides of nitrogen. In order to reduce the proportion of these components to a minimum demanded by the authorities, the exhaust gases must be substantially freed from these materials. This means that the combustible components must be oxidized as completely as possible to carbon dioxide and water, and the oxides of nitrogen must be reduced to nitrogen.
Such a conversion is carried out by subjecting the exhaust gases to an aftertreatment over catalysts. It is, however, a precondition for optimum operation of the catalyst that the fuel/air mixture burnt in the engine should approximately correspond to the stoichiometric ratio between air and fuel (.lambda.=1). For this reason, an electro-chemical sensor (.lambda. probe) is installed in the exhaust path befor the catalyst to measure the oxygen content in the gas (for example).
A control device processes the signal emitted by the probe and adjusts the setting of the fuel/air ratio and hence also, to a large extent, of the exhaust gas composition. Such so-called control systems have been known for a long time and, theoretically, they operate satisfactorily. Aging phenomena do, however, appear and these make it impossible to adjust to an optimum mixture as the operational life increases, so that faulty adjustments occur.
According to the U.S. Pat. No. 4,622,804 these faulty adjustments can be corrected by installing a second .lambda. probe after the catalyst. The post catalyst probe has a substantially smoother signal relative to that of the pre-catalyst probe and permits simple determination and correction of the control working point. The correction takes place by determining the amplitude and mean value of the output signal of the post-catalyst probe. If the mean value differs from a specified set value, the control working point is altered until post-catalyst probe again achieves its set value. The use of this post-catalyst probe makes it possible to smooth out those particular control errors which are due to aging of the pre-catalyst probe or of the fuel measuring system or which are caused by an excessive hydrogen content in the exhaust gas, which leads to a displacement of the .lambda. characteristic.
Although mixture control by the use of post-catalyst probes operates extremely satisfactorily even for long periods, they are inaccurate in the non-steady range, i.e. during dynamic transition from one steady operating condition to a different one, for example during acceleration and braking phases. In actual driving circumstances, this leads to an undesirable deterioration in the exhaust gas parameters.
The object of the invention is to find a control method for optimizing the fuel/air ratio in the non-steady condition in an internal combustion engine which is equipped with a .lambda. probe before and a .lambda. probe after the exhaust gas catalyst, whereby the undesired exhaust gas components are minimized even under non-steady conditions.
This object is achieved by means of the control system an method which provides a correction signal under non-steady operating conditions and iterates the signal to produce the optimum correction requiring the minimum amount of time to achieve the desired fuel/air ratio. The control system is an improvement in a control system having a pre- and post-exhaust catalyst .lambda. probe and adjusts the fuel/air ratio to achieve a desired fuel/air ratio compared to that sensed by the probes. The method includes storing a plurality of sets of control values and time values of the duration which the post-catalyst probe would indicate an undesirable fuel/air ratio for a plurality of sets of specified engine operating conditions in the non-steady range of operation. Once a non-steady range of operation is determined, the specific engine operating conditions are sensed and a corresponding set of a correction control value and a time value are read. The control system modifies the read correction control value as a function of the read time to achieve a desired fuel/air ratio. The time period which the post-catalyst probe indicates an undesirable fuel/air ratio is measured and compared to the read time value. If the measured time value is less than the read time value, the modified correction value and the measured time are stored for the set of sensed engine operating conditions. This process is repeated each time a specific set of engine operating conditions are met. The specific engine operating conditions may include the rotational speed of the engine and the air flow mass which may be measured by deflection of a sensor plate located in the induction ducting. The non-steady range includes acceleration and deceleration. The modification of the correction control values includes determining whether the fuel/air ratio is rich or lean and modifies the correlation based upon this determination and the read time. The non-steady state correction system requires a minimum level of non-steady state range operation before providing a correction signal.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.